Re: [PATCH v3 3/8] KVM: arm64: Fix MTE flag initialization for protected VMs

[Ben Horgan <ben.horgan@arm.com>]

Hi Fuad,

On 11/10/25 13:45, Fuad Tabba wrote:
> The function pkvm_init_features_from_host() initializes guest
> features, propagating them from the host. The logic to propagate
> KVM_ARCH_FLAG_MTE_ENABLED (Memory Tagging Extension)
> has a couple of issues.
> 
> First, the check was in the common path, before the divergence for
> protected and non-protected VMs. For non-protected VMs, this was
> unnecessary, as 'kvm->arch.flags' is completely overwritten by
> host_arch_flags immediately after, which already contains the MTE flag.
> For protected VMs, this was setting the flag even if the feature is not
> allowed.
> 
> Second, the check was reading 'host_kvm->arch.flags' instead of using
> the local 'host_arch_flags', which is read once from the host flags.
> 
> Fix these by moving the MTE flag check inside the protected-VM-only
> path, checking if the feature is allowed, and changing it to use the
> correct host_arch_flags local variable. This ensures non-protected VMs
> get the flag via the bulk copy, and protected VMs get it via an explicit
> check.
> 
> Fixes: b7f345fbc32a ("KVM: arm64: Fix FEAT_MTE in pKVM")
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/pkvm.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> index f6f8996c4f97..7e370e31260d 100644
> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> @@ -337,9 +337,6 @@ static void pkvm_init_features_from_host(struct pkvm_hyp_vm *hyp_vm, const struc
>  	/* CTR_EL0 is always under host control, even for protected VMs. */
>  	hyp_vm->kvm.arch.ctr_el0 = host_kvm->arch.ctr_el0;
>  
> -	if (test_bit(KVM_ARCH_FLAG_MTE_ENABLED, &host_kvm->arch.flags))
> -		set_bit(KVM_ARCH_FLAG_MTE_ENABLED, &kvm->arch.flags);
> -
>  	/* No restrictions for non-protected VMs. */
>  	if (!kvm_vm_is_protected(kvm)) {
>  		hyp_vm->kvm.arch.flags = host_arch_flags;
> @@ -372,6 +369,11 @@ static void pkvm_init_features_from_host(struct pkvm_hyp_vm *hyp_vm, const struc
>  		kvm->arch.flags |= host_arch_flags & BIT(KVM_ARCH_FLAG_GUEST_HAS_SVE);
>  	}
>  
> +	if (kvm_pvm_ext_allowed(KVM_CAP_ARM_MTE)) {
> +		set_bit(KVM_CAP_ARM_MTE, allowed_features);

Isn't allowed_features a bitmap for the KVM_ARCH_VCPU_ defines? I think
MTE is treated as a property of the VM.

> +		kvm->arch.flags |= host_arch_flags & BIT(KVM_ARCH_FLAG_MTE_ENABLED);
> +	}
> +
>  	bitmap_and(kvm->arch.vcpu_features, host_kvm->arch.vcpu_features,
>  		   allowed_features, KVM_VCPU_MAX_FEATURES);
>  }


Thanks,

Ben